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Where the Brain Monitors the Body

As any klutz will tell you, coordination is complicated. Just walking down the street--let alone juggling--requires large portions of the brain to keep track of the body and move its limbs. Now neuroscientists think they've found a brain area that performs one of the more difficult of these chores--integrating sensations to figure out the body's position.

People and animals keep track of their limbs with several senses: vision; touch; proprioception, or the feel of the body's position; and memories of recent movements. Sometimes integrating these signals goes awry. For instance, some people with damage to a part of the brain called the parietal cortex don't recognize a limb as their own. They might wake up startled, thinking someone has put a fake leg in their bed. It's puzzling what goes wrong in these people--and equally mysterious how the senses cooperate in a healthy brain.

To locate where signals from vision and proprioception come together in the brain, a team led by psychologist Michael Graziano of Princeton University first had to tease apart these streams of information in monkeys. The researchers hid the monkey's arms beneath a shallow ledge and placed a realistic, stuffed monkey arm on top of the ledge where the monkey could see it. This way the researchers could measure whether signals are caused by the monkey looking at an arm, versus feeling it.

The team tested neurons in a region of the parietal cortex called area 5 that's involved in proprioception. They recorded firing rates from neurons that responded selectively to one arm or another. Then they checked to see if vision plays a role by examining whether the neurons changed their firing rates when the monkey saw the fake arm. If the fake arm was put on the wrong side of the body or flipped around so that the palm was toward the body, the neurons didn't respond. But 29% of neurons did change their firing rates when the visible fake arm was put in a position similar to that of the felt--but hidden--real arm. As the researchers report in the 30 November issue of Science, these neurons appear to be sensitive to both vision and proprioception.

The study suggests that "the information processed by area 5 is more multisensory, more abstract" than simple proprioception, says neurophysiologist Lawrence Snyder of Washington University in Saint Louis. And if area 5 neurons integrate signals from many channels, Snyder says, they might be the first stages of a "representation of where the body is in space."